1. Field of the Invention
This invention relates to the making of semiconductor devices using lithographic masks to expose circuit patterns on the device, and, more particularly, to lithographic masks and a method of making lithographic masks such as a stencil mask which are used with electron beams, ion beams or photon beam lithography techniques to pattern the device.
2. Description of Related Art
Exposure masks are used in lithographic techniques during the manufacture of integrated circuits to define circuit features on a wafer. During construction of the mask, a pattern of the circuit features to be exposed onto the wafer is formed on the mask substrate. The mask is placed in a position to overlie the semiconductor wafer and an energy beam passes through the mask which exposes a suitable sensitized film covering the wafer in the pattern on the mask. When the wafer is exposed, the pattern on the mask is reproduced in a sensitized film. Subsequent processing, such as developing the sensitized film, etching, etc. in accordance with a particular lithographic technique being used produces the circuit pattern on the mask on the surface of the wafer to define the desired circuit patterns.
The tendency of integrated circuits in semiconductor technology, however, is to ever decreasing structured dimensions in order to increase the density of the circuits and their switching speed. Optical photo lithography, which is still used today in the majority of cases, is approaching the limits dictated by the physical resolution of optical systems. The most promising methods for the production of devices having finer lines and circuits are x-ray and electron beam processes which employ stencil masks.
With continued advances in the miniaturization of circuit patterns, and the consideration of the resolution limitations of light, much recent research on microlithography has been directed to the use of shorter-wave length electromagnetic radiation such as x-rays or charged-particle beams such as electron beams, ion beams, etc. rather than visible or UV light. Such masks generally comprise a silicon mask substrate with defined patterned through-holes in the substrate. The pattern defined by the array of holes or voids is transferred to the sensitive substrate of the semiconductor device and such masks are conventionally termed xe2x80x9cstencil masksxe2x80x9d.
In the formation of stencil masks especially for advanced lithography techniques such as electron beam, ion beam or photon beam lithography, typically the membrane is defined first then the patterns are etched into the membrane. This order is chosen for stress and image placement control and for membrane etch yield issues. However, this prevents the pattern etch (if performed with a reactive ion etch) from using any backside cooling of the membrane. The lack of backside cooling usually leads to loss of control of critical dimensions. An additional disadvantage of prior art methods is pattern profile degradation such as notching or tapering near the bottom edge of the membrane.
A number of patents have issued with regard to mask fabrication including U.S. Pat. Nos. 5,858,576; 5,876,881; 5,899,728; and 5,905,005. All of the above patents are hereby incorporated by reference.
In the prior art methods for forming stencil and lithographic masks backside cooling of the membrane is limited and bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for making stencil and other lithographic masks used in the manufacture of semiconductor devices.
It is another object of the present invention to provide lithographic masks including stencil masks made by the method of the invention.
A further object of the invention is to provide semiconductor devices made using the lithographic mask of the invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method of making a stencil or other lithographic mask comprising the steps of:
obtaining a lithographic mask substrate having an upper surface and a lower surface;
etching the lower surface of the substrate to form window openings therein;
depositing a protective layer over the lower surface and window openings of the etched substrate;
etching the upper surface of the substrate forming the desired stencil pattern in the substrate; and
removing the protective layer forming the mask.
In a preferred aspect the substrate has a stencil pattern forming layer on the upper surface thereof (which layer may be also termed a membrane layer) which layer is etched to form the stencil pattern therein after the protective layer is applied.
In another aspect, a method of making a stencil or other lithographic mask is provided comprising the steps of:
obtaining a lithographic mask substrate comprising sequential layers of, from bottom to top, a backside mask layer, a window forming substrate layer, a stencil pattern forming layer and optionally a frontside mask layer, each layer having a lower surface and an upper surface;
etching the backside mask layer to form one or more patterned backside openings;
etching the window forming substrate layer to form window openings therein and exposing a portion of the lower surface of the stencil pattern forming layer;
depositing a protective layer over the etched backside mask layer and etched window forming substrate layer and the exposed portion of the lower surface of the stencil pattern forming layer;
depositing an imageable resist layer on the stencil pattern forming layer upper surface or on the optional frontside mask layer upper surface if used;
patterning the imageable resist;
etching the stencil pattern forming layer or the optional frontside mask layer forming the pattern on the stencil pattern forming layer or optional frontside mask layer;
etching the stencil pattern forming layer in the pattern on the frontside mask layer if a frontside mask layer was used;
removing the resist layer; and
removing the protective layer forming the mask.
In another aspect of the invention, a method is provided for making a lithographic mask such as a stencil mask comprising the steps of:
obtaining a lithographic mask substrate having an upper surface and a lower surface;
etching the upper surface of the substrate forming the stencil pattern;
depositing a frontside protective layer on the etched substrate upper surface;
etching the lower surface of the substrate forming window openings in the substrate; and
removing the frontside protective layer forming the mask.
In another aspect, the substrate has a stencil pattern forming layer on the upper surface thereof which layer is etched to form the stencil pattern therein and then the frontside protective layer deposited and the method continued.
In a further aspect of the invention, a method is provided for making a lithographic mask such as a stencil mask comprising the steps of:
obtaining a lithographic mask substrate comprising sequential layers, from bottom to top, of a backside mask layer, a window forming substrate layer, a stencil pattern forming layer and optionally a frontside mask layer, each layer having a lower surface and an upper surface;
etching the backside mask layer to form one or more patterned backside openings;
depositing an imageable resist layer on the upper surface of the stencil pattern forming layer or the optional frontside mask layer;
patterning the imageable resist layer;
etching the stencil pattern forming layer or the optional frontside mask layer forming the pattern;
etching the stencil pattern forming layer in the pattern on the frontside mask layer if a frontside mask layer was used;
removing the resist layer;
depositing a frontside protective layer on the etched stencil pattern forming layer surface;
etching the window forming substrate layer forming window forming substrate layer openings and exposing a portion of the lower surface of the patterned stencil pattern forming layer; and
removing the frontside protective layer forming the mask.
In another aspect of the invention a method is provided for making semiconductor devices using the lithographic masks of the invention wherein the mask is positioned to overlie the semiconductor device to be patterned and an energy beam passed through the mask and directed to the surface of the semiconductor device which is covered with a sensitive resist. The resist is thereby exposed in the desired pattern and after conventional techniques of developing and etching the resist, the desired circuit pattern is formed on the semiconductor device.
In a further aspect of the invention semiconductor devices made using the method of the invention are provided.